Poliovirus infection of cells in tissue culture results in a rapid shut-off of host protein synthesis by a mechanism that appears to be relatively complex. The first step involves activation of a protease, the second is the specific cleavage of the cellular cap- binding complex subunit, p220, and the third may be the specific stimulation of viral mRNA translation. Cleavage of p220 results in an inhibition of cap-dependent translation, but the specific function of p220 that is altered by cleavage is unknown. This proposal will focus on the function of p220 and the changes in that function which occur as a result virus-induced cleavage. Preliminary results suggest that the function of p220 is related to stabilization of complexes involved in the mRNA-binding step of protein synthesis. Monoclonal antibodies will be used to detect p220 and other initiation factor polypeptides in experiments designed to determine the effects of in vitro cleavage of p220 on the composition of cap affinity-purified complexes. Cap-binding complexes containing cleaved and uncleaved p220 will also be analyzed for various in vitro translation activities. Poliovirus mRNA-binding proteins will be identified in an effort to determine whether altered cap-binding complexes are involved in the specific stimulation of poliovirus mRNA translation. This approach will begin an examination of the role of these factors in cap- independent translation. The structure of p220 and the cleavage products will be analyzed by peptide mapping and selected peptide sequencing in preparation for the synthesis of oligonucleotides to use as screening probes for gene cloning purposes. Finally, an analysis of the structure and modification of cap-binding complexes of L cells before and after Mengovirus infection will provide comparative information regarding the mechanism of host cell shut- off by picornaviruses which do not induce p220 cleavage.